A packet transfer system using a proxy device has been proposed (for example, see Patent Document 1). In the system of Patent Document 1, when a transmission network device 1 and a reception network device 4 transmit and receive packets according to Transmission Control Protocol/Internet Protocol (TCP/IP), a TCP server 2 and a TCP server 3 transmit and receive packets according to TCP/IP protocol on behalf of the transmission network device 1 and the reception network device 4. Accordingly, it possible to efficiently transmit and receive packets in the transmission network device 1 and the reception network device 4.